Electrical insulating oils and method



United States atent O ELECTRICAL INSULATING OILS AND METHOD FOR PRODUCING THEM Mintje van Loon, The Hague, and Govert Verberg,

Amsterdam, Netherlands, assignors to Shell Development Company, New York, N. Y., a corporation of Delaware No Drawing. Application September 4, 1956 Serial No. 607,580

Claims priority, application Netherlands November 15, 1955 3 Claims. (Cl. 208211) This invention relates to improved insulating oils and more particularly to a process for preparing electrical insulating oils which are stable, non-sludging and resistant to gassing.

It is known that when hydrocarbon insulating oils are subjected to high electrical stress, they tend to decompose producing sludge and gaseous products, which may cause serious damage and eventual failure of the apparatus in which they are used. The presence of aromatic hydrocarbons in the oil tends to inhibit gassing, but they have an adverse effect in that they increase sludging. Drastic refining of highly aromatic oils in order to produce more stable electrical insulating oils results in poor yields, generally in the neighborhood of 45-55% by weight, calculated on the starting material.

It has now been found that non-gassing, sludge resistant and oxidation stable electrical insulating oils can be prepared in high yields from mineral oils'having a viscosity of between 1 and 9 Engler (E) at 20 C. and preferably between 3 and 5 rich in aromatics and relatively high in sulfur content, namely 1% by weight or more.

By aromatic rich oils it is meant that the aromatic content of the oil is 35 or more as determined by percolation over silica gel as described in the Journal of the Institute of Petroleum, 36, 89-104 (1950). t

A process for the production of a non-gassing and oxidation-stable electrical insulating mineral oil composition, which comprises:

(a) Separately (1) catalytically selectively hydrogenating a portion of an oil '(A) and (2) selectively solvent extracting a portion of an oil (B), oils (A) and (B) each being mineral oils having viscosities between 1 and 9 Engler at 20 35% by weight and a sulfur content of at least 1% by weight, and

(b) Subsequently separately treating the thus-treated portions of oil (A) and of oil (B), with strong sulfuric acid, removing the sludge resulting therefrom, neutralizing the acid treated oil and thereafter treating the thus separately neutralized oils with a solid absorbent, the resulting thus completely-treated portion of oils (A) and (B) being blended in proportions of from 80 to 40 parts by weight of oil (A) and from 20 to 60 parts-by weight of oil (B). Oil portions (A) and (B) should be treated separately by step (b) and not combined and thereafter processed by step (b). If the two portions E at 20 C. and which are C., an aromatic content of at least (A) and (B) are treated by step (b) together, the final 1 aromatic content of at least 35 by weight and a sulfur content of at least 1% by weight, by, (1) first catalytically selectively hydrogenating this oil to a sulfur content of from 0.050.06% by weight, then (2) treating it with sulfuric acid or oleum, (3) separating the acid sludge, (4) neutralizing the acid oil and (5) treating it with a solid absorbent; s I

(b) Preparing an oil (B), starting from a mineral oil having a viscosity of between 1 and 9 E at 20 .C., an aromatic content of at least 35 ,by weight and a sulfur content of at least 1% by weight, by (1) first extracting this oil with a selective solvent .for the aromatic components of the oil to produce a raflinate with an aromatic content of not more than 25% by weight, (2) treating the raflinate obtained with concentrated sulfuric acid or oleum, (3) separating the resulting acid sludge, (4) neutralizing the acid oil obtained and, (5) treating it with the invention in separately processing the portions ofoil A and of oil B, with a final blending of the thus-treated portions A and B, will give a better understanding of its practice.

PREPARATION OF OIL (A) For preparing oil (A) the starting oil is first catalytically selectively hydrogenated to a sulfur content of 0.05- 0.6% by weight, preferably 02-03% by weight, ,and especially approximately 0.25% byweight. By catalytic selective hydrogenation is here meant a catalytic hydrogenation under conditions in which the non-hydrocarbons as sulfur or oxygen) are hydrogenated, but the hydrocarbons remain essentially unaifected (so that,.for example, there is virtually no cracking of the oil or hydrogenation of aromatics present in the oil). Several methods are already known (see for example, Erdol and Kohle, 6, 616-619 (1953)) for hydrogenating oils in this way, oil may be in either the gas or the liquid phase during the hydrogenation. In general, the selective hydrogenation is carried out at a temperature of 300 C.-400 C a pressure of 10-200 kg. per sq. cm., a gas discharge rateof 50-5,000 liters per kg. of oil and a flow rate of the oil of 0.3-3 kg. per liter of catalyst per hour. Suitable catalysts are oxygen or sulfur-containing compounds such as oxides and sulfides of metals of groups VI and VIII of the periodic table. Mention may be made of cobalt oxide, molybdenum oxide, tungsten sulfide and nickel sulfide. Thecatalysts may be supported on carriers,'such as active carbon, fullers earth, kieselguhr, silica or alumina, e. g. in the form of bauxite, pumice or burnt clay. A very suitable catalyst consists of cobalt oxide and molybdenum oxide on bauxite as carrier. Suitable hydrogenation methods are described, for example, in

the British patent specifications 657,521, 665,575 and In the catalytic selective hydrogenation, hydrogen sulfide isformed from the sulfur compounds present in the oil. A part of the hydrogen sulfide formed dissolves in the oil, and this should be removed from the oil before the treatment with concentrated sulfuric acid or oleum.

This can be eflected, for example, by Washing the oil with a caustic alkali solution, but it is most simply carried out by blowing through the oil an inert gas such as nitrogen, if desired, at elevated temperature.

which is frequently termed hydrofining. The- Electrical insulating oils are often required to have a I flash point which. is not below a certain. temperature. As.

a result of the catalytic selective hydrogenation, the initial boiling point of the oil is somewhat reduced. This causes a lowering of the flash point in the oil. As a result, the flash point of the final insulating oil may not satisfy the flash point requirement. If this isthe: case, the lows; boiling compounds should be removed from the '1. This can be done in a simple way by means of, distillat iqn, preferably under reduced pressure Distillation is, carried. out until the residue has the d sired flash point. The removal of the low-boiling compounds is preferably carried out before the treatment with; concentrated sulfuric acid or oleum.

The treatment with concentrated sulfuric acid or oleum is carried out in a manner known per se. For instance, concentrated sulfuric acid (SQ-100%) or oleum (to 30% S may be used in amounts of from 5-30% by weight, calculated on the oil treated, at temperatures varying from 0 to 75 C. and contact times from 5 sec. to one hour. The refining can be carried out in one or more stages and either batchwise or continuously. In batchwise refining the treatment temperature is preferably approximately 25 C., and in continuous. refining preferably approximately 60 C. After the treatment with concentrated sulfuric acid or. oleum, the acid sludge formed is removed from the oil.

'The oil which may still contain residual acid or sulfonic acids formed during the acid treatment, is then treated with an alkaline material such as an aqueous or alcoholic solution of NaOH in order to neutralize the oil. The used alkaline material is separated from the oil, and the oil subsequently washed, if desired, e. g. with water and/ or alcohol.

Finally the oil is, treated with a solid adsorbent such as, adsorbent earths and clay types Suitable adsorbents are, for example, fullers earth, bauxite and clay such as Filtrol, whether. or not activated with acid. Small quantities of alkaline material such as lime or magnesia may be added to the adsorbent. If desired, the oil may be heated during the treatment, and an inert gas such as nitrogen passed through the oil. The quantity of adsorbent used generally liesv between 1 and 30% by weight, calculated on the oil. treated.

When the selectively hydrogenated oil is stored for a relatively long period befo e the acid treatment, dark sludgy deposits. may be formed in the oil. These deposits may be formed after 1-2 months, but sometimes also before or after. The formation of these deposits can be prevented by washingthe oil with an alcoholic caustic alkali solution, e. g. a 4% by weight solution of NaOH in 40% by weight alcohol, after the selective hydrogenation but before storage. This. treatment may also be used at the same time to remove from the oil the hydrogen sulfide formed in the catalytic selective hydrogenation and dissolved in the oil. i

PREPARATION OF OIL (B) For preparing oil (B) the same starting oil may be used as in the preparation of oil (A), or another oil, provided the starting material satisfies the foregoingrequirements.

The starting material is first extracted in. a manner known per so with a selective solvent for the aromatic components of the oil (e. g. liquid sulfur dioxide, furfural, phenol, nitro benzene, etc.) to produce a rafiinate with.

an aromatic content of not-more than 25 by weight and preferably no more than by weight.

This raffinate isthen treated with concentrated sulfuric acid or oleum and then subsequently treated as described for the preparation of oil (A). A

It has already been pointed out the process of the invention'as compared. to the. known, processes is that a larger yieldof final insulating oil is obtained. A further advantage ofthe process of the in vention is that the insulating oilsobtained have improved gassing characteristics. The processof the invention is.

that an advantage of of particular advantage when the starting oils have an aromatic content of 40% by weightor more, and particularly a sulfur content of 1.5% by weight or more.

The advantages of the process of the invention as compared to the known processes are shown by Examples 1 and II.

Example I PREPARATION OF OIL A I The starting material was a spindle oil obtained by distilling and re-distilling over lime a Venezuelan naphfinal electrical insulating oil was thenic crude oil. The. spindle oil distillate had the following properties:

Viscosity 5.5 E./20 C. Flash point (Pensky-Martens, closed) 156 C.

Sulfur content 1.76% by weight. Aromatic content 50% by weight.

This oil was catalytically selectively hydrogenated by allowing the oil to flow in a thin layer over a catalyst bed of cobalt oxide and molybdenum oxide supported on bauxite in the form of cylindrical tablets measuring 5 X 5 mm. under the following conditions:

Temperature, C 370 Hydrogen pressure, kg./sq. cm 50 Flow rate of the oil, kg./liter, catalyst/hour 1.1 Gas discharge rate, liter/kg. oil 225 Approximately 2% by weight of the oil was lost in the catalytic selective hydrogenation.

The hydrogen sulfide dissolved in the oil was then removed by blowing with nitrogen at 60 C. C.

The oil was then subjected to fractional distillation at a pressure of 20- mm. Hg until the temperature of the vapor passing over was C. Approximately 6% by weight of the oil was thus removed.

The. flash point of the oil was now 148 C. The sulfur content of the oil had dropped to().25% by weight, 'while the aromatic content was practically unchanged, as is shown from the very slight change in the refractive index (n =l.5018; for the starting material n :l.5l03).

10%. by weight of oleum (17% by weight of 80 was now added to the oil at a temperature of 20 C. and the mixture was then stirred for /2 hour. The acid sludge was then allowed to deposit. This was separated off and the oil was subsequently treated with a 4% solution of NaOH in 40% alcohol, and afterwards washed with dilute alcohol and water. Finally the oil was treated with 3% by weight of Filtrol for /2 hour while passing nitrogen through it.

The. amount of oil obtained in this way was 79% by weight calculated on the starting material.

PREPARATION OF OIL B I Another portion of the same starting oil was used as in the preparation of oil A I. This oil was extracted with liquid sulfur dioxide to produce a Iafli'nate with anaromatic content of 9% by weight. The raffinate was treated with 10% by weight of oleum and then aftertreated, i. e., neutralized and clay contacted as described in the preparation of oil A I.

The. amount of oil obtained in this was 43.5% by weight, calculated on the starting material.

MIXING OILS A I AND B I, AND PROPERTIES OF FINAL ELECTRICAL INSULATING OIL of finished oil A I were mixed with finished oil B I. The total yieldof by weight, calculated on the total amount of starting material.

The oxidation stability of the final, oil was determined in the British Standard. Specification Test for Insulating 60 parts by weight 40 parts by weight of Oils (B. S. 1.) According to the specification, after the conclusion of the test, the limit of the sludge formation in the oil. should be l .l% by weightand the maximum acid nurnber of the oillimg. of KOH/g. of oil.

5. The final electrical insnlatingoil amply satisfied this specification:

The gassing characteristics of the final oil were evaluated in the modified Pirelli apparatus (Journal of the Institute of Petroleum, 35, No. 311, pp. 735-754) at a temperature of 50 C. under a hydrogen atmosphere and at a voltage of kilovolts. The amount of hydrogen which the oil adsorbs or evolves under these conditions in the course of time is measured. The hydrogen absorption or hydrogen evolution expressed in millimeters of oil pressure is plotted in a graph as a function of the time and the slope of the line thus obtained determines the gassing coefiicient of the oil. For the present oil this coeflicient was found to have the value of -2.8, which is very favorable.

Example II In the B. S. I. test the oil gave the following results: Sludge, percent by weight 0.84 Acid number 1.4

The yield was 55% by weight, calculated on the startingmaterial, and the gassing coeflicient of the oil was positive.

Example I shows that the high yield in the process of the invention is chiefly due to the fact that in the preparation of oil A, relatively little of the starting material is lost (yield of oil A I is 79% by weight).

The improved gassing characteristics of the insulating oils of the invention are probably to be explained by the fact than in preparing oil (A) the aromatic content of the starting material is very little reduced so that the final insulating oil (despite the fact that another oil (B) with a considerably lower aromatic content is added), will have a higher aromatic content than an insulating oil prepared by the conventional method. An indication of this is that the insulating oil according to the invention has a higher refractive index than the insulating oil prepared in the conventional manner (11 1.4851 and 1.4784, respectively).

It is necessary to add an oil (B) to oil (A) because an oil (A) has an il'lSlJfilClCIlt oxidation stability in itself. As was already observed above, in the preparation of oil (B) the starting oilis extracted in order to obtain a rafiinate with an aromatic content of not more than 25% by weight, and preferably 15% by weight or less. The degree of extraction in the preparation of oil (B) affects the oxidation stability of the final insulating oil to be obtained as shown by Example III.

Example III Oil s B I and B II both improve the oxidation stability of oil A I, but oil B I gives a greater improvement than oil B H, as is shown by the following table:

B. S. I. test 011 Sludge, Acid percent number by weight As was already observed above, in the preparation of oil (A) the starting oil is catalytically selectively hydrogenated to a sulfur content of 0.05-0.6% by weight and preferably to 0.20.3% by weight. The degree of catalytic selective hydrogenation affects the oxidation stability of the final insulating oil to be obtained, as is shown by Example IV.

' Example IV The same starting material was used as in Example I. Oils A II and A III were prepared from two portions of this starting material in the manner described in Example I for the preparation of oil A I, except that the conditions in the catalytic selective hydrogenation were An oil B II was prepared from the same starting masomewhat modified, so that the hydrogenation products have a different sulfur content. The conditions in the catalytic selective hydrogenation were as follows:

A II A III Temperature, C 375 375 Hydrogen pressure kgJsq. cm 50 50 Flow rate of the 011 kg./liter catalyst/hour 0.5 2.0 Gas discharge rate, liter/kg. oil 260 The sulfur content of the 7 oils, .after blowing nitrogen S-content B. S. I. test of oil (A) Oil after sel.

hydr. in Sludge, Acid percent percent number by weight bywcight;

It is found that the mixture with'oil A I amply satisfied the specification and the mixtures with oils A II and A III only just satisfied it.

Thefollowing tableshows that by adding larger amounts of oil B I, the oxidation stability of oil A III may be still further improved:

B. S, I. test;

' Oil Sludge, Acid v percent number byweight 60% A III 1.05 1. 5 50% A III 0.92 1. 4 40% A III 0. 82 1. 5

The fact that oil fractions (A) and (B) must be treated separately and not together with sulfuric acid or oleum, the acid sludge removed, the acid oil neutralized 7" nd e f r: tre ted with; a. solid adsorben is illusat d by e fo lo ng. examp e...

Example IVA The same starting material as used in Example I was hydrofined' to a sulfur content of 0.23% by weight (oil X), Another portion of the same starting material was extracted to an, aromatic content of 9% by weight, as described in Example I under the heading Preparation of Oil BI (oil Y'). Oil's X and Y were mixed in a weight ratio of 60:40 and the mixture was treated with 120% oleum at C; and treated further as, described in. Example I. The final product was tested in the B. S. I. test.

Test; results: 1.17%. by: weight of sludge; 118 acid number.

It: has already beenobserved: above that the high yield which can be, obtained with the process of the invention is chiefly due. tov the fact; that in the preparation of oil (A). relatively'little ofthe. starting: material is lost The yield of final insulating oil is consequently lower ac.- cording as more of anoil-z B)? is. used. Actually, the yield. in the cases. referred to in the above table was 6'5 61; and 58% by weightrespectively, calculatedout-he starting material, Acomparison with Example II shows that the use ofmore than 6. by weight oi anoil (B) in the mixture gives scarcely any better yield than obtained according to the conventional method.

Insulating oils prepared, by the process of this invention possess. satisfactory oxidation and electrical characteristics. If desired, however, minor proportions of from about 0.01% to about 1% as for example, 0.3 percent by weight, of a suitable anti-oxidant may be incorporated, such as alkyl phenols exemplified by 2,6-di tert-butyl? 4 methylphenol' or 2,4edirnethylg6stert-butylgv phenol.

The good anti-oxidant sensitivity of the insulating oils ofthis invention is shown in Example V;

Example V 0.3% by: weight of 2,6edi tert-butyl-4-methylphenol was added as an anti-oxidant tothe insulating-foil described in Example I, and the oil was tested in the B. S. I. test. The results. (as compared to the oil without antioxidant) are shown in the following table:

Example VI The. oxidation stability of the insulating oil of Example, I was. also tested in the-,S. E. V. test (see- Zerbes book, Mineralole and verwandte; Produkte, p. 942 According to the specification after atest period; of 7 days not more than 0.15% by: weight of sludge should be: formed inthe oil. Moreover the acid number; of the. oil after a test period of 3; days should not be more than 0.3, and after a test period of 7 days, not more than 0.4. Finally the tensile strength of the cotton threads in the oil during the test should not have decreased by more than after a test period of 3 days, and by not more than after a test period of 7 days.

The oil more than satisfied this specification:

Example VII;

From the same starting material as used in Example I" an oil A IV" was prepared in the manner described for oil A I in Example 1, except that in the acid refining 25% by weight of sulfuric acid of 96% was used instead of parts by weight of oil B-L (see Example L). The total yield of final insulating oil was 68% by weight. The oil gave. the; following results inthe B.S.I.. test;

B S; I. test Sludge, 3 Acid percent number by weight 70%.A 1v+ a0% B 1 0.95 1.1

Minor amounts. of. from 0.01% to 1% of still other classes of conventional additives can, be incorporated, in oils of this invention such as corrosion inh bitorsgas sing inhibitors, sludge and carbflninhibitors, e. g. oil-soluble compounds capable of chelating metal ions, cycloalkene hydrocarbons, aromatic amines and halogenated aromatichydrocarbons and mixtures thereof.

We claim asour invention:

1-'. A process for the production of a non-gassing and oxidation stable electrical insulating oil from a mineral oil containing at least- 35 aromaticsand' 1% sulfur and having a viscosity between 1 and 9 Engler at 20 C. comprising splitting the oil into portions- (A) and (B), separately treating oil portion (A) by catalytically selectively hydrogenating the oil tosubstantially reduce; the: sulfur content without essentially etfecting the aromatic content of the oil and separately treating oil portion (B) with a selective aromatic, solvent to obtain a rafiinate fraction and subsequently treating these two oil portions,( A). and (B) with sulfuric acid, removing the sludge from the acid treated oil portions (A) and (B), neutralizing the acid treated oil mixture (A) and (B) and thereafter treating the neutralized oil mixture (A) and (B) with a solid absorbent, the treated products of; oil portion (A) and oil portion (B) being blended in proportion of from to 40 parts by weight.

of oil (A) with from 20 to:60 parts by-weight of oil (B).

2. A process according to claim 1, wherein the base oil is a mineral oil having a viscosity of between land 9 E. at 20 (3., an aromatic content of at least 35% and a sulfur content of at least 1% by weight and wherein the sulfur content of oil (A) is reduced to about, 0.2 to 0.3% byweight and the aromatic content being substantially unchanged and the oil (13') aromatic, content being reduced to less than 25 byweight.

3. An electrical insulating oil prepared by the process of claim 1 w i I RefereneesCited in thefile of this patent.

UNITED STATES PATENTS 1,881,534 Harding: Oct. 11, 1932 2,087,578 Nederbragt July'20, 1937 2,574,451 Porter et a1. Nov. 6. 1951 2,734,019 Miller et al. Feb. 7, 1956 2,754,248 Wetzel July 10, 1956 

1. A PROCESS FOR THE PRODUCTION OF A NON-GASSING AND OXIDATION STABLE ELECTRICAL INSULATING OIL FROM A MINERAL OIL CONTAINING AT LEAST 35% AROMATICS AND 1% SULFUR AND HAVING A VISCOSITY BETWEEN 1* AND 9* ENGLER AT 20*C COMPRISING SPLITTING THE OIL INTO PORTIONS (A) AND (B), SEPARATELY TREATING OIL PORTION (A) BY CATALYTICALLY SELECTIVELY HYDROGENATING THE OIL TO SUBSTANTIALLY REDUCE THE SULFUR CONTENT WITHOUT ESSENTIALLY EFFECTING THE AROMATIC CONTENT OF THE OIL AND SEPARATELY TREATING OIL PORTION (B) WITH A SELECTIVE AROMATIC SOLVENT TO OBTAIN A RAFFINATE FRACTION AND SUBSEQUENTLY TREATING THESE TWO OIL PORTIONS (A) AND (B) WITH SULFURIC ACID, REMOVING THE SLUDGE FROM THE ACID TREATED OIL PORTIONS (A) AND (B), NEUTRALIZING THE ACID TREATED OIL MIXTURE (A) AND (B) AND THEREAFTER TREATING THE NEUTRALIZED OIL MIXTURE (A) AND (B) WITH A SOLID ABSORBENT, THE TREATED PRODUCTS OF OIL PORTION (A) AND OIL PORTION (B) BEING BLENDED IN PROPORTION OF FROM 80 TO 40 PARTS BY WEIGHT OF OIL (A) WITH FROM 20 TO 60 PARTS BY WEIGHT OF OIL (B). 